The testing of earthen samples is a common practice to obtain accurate values pertaining to the strength and stability of the underlying materials under loading. In order to obtain accurate values it is necessary to test samples of materials under conditions simulating, as closely as possible, natural conditions. Several practical problems involving soils under or adjacent to embankments, cuttings, retaining walls, piles and offshore platforms involve soil behavior that can be closely simulated in the laboratory by subjecting a sample of soil or other earthen materials and powders to a simple shear test. There are several types of devices which can be used to test various characteristics of soils and other samples.
The present invention involves a type of device called the simple shear device. A simple shear device will test a sample of particulate geologic or mineralogic material under conditions of plane strain (a condition in which no strains occur in one of the Cartesian axes) with rotation of the principal axes of stress and strain. One of the advantages of a simple shear device is that it allows the principal axes of stresses and strains to rotate freely. A general soil element in the ground can expect to experience varying directions and magnitudes of principal stresses and it becomes important to study the behavior of soil samples in the laboratory under stress conditions in which rotation of principal axes can occur freely. In addition, simple shear devices distort as well as compress the sample. Shear can be simply defined as a straining action wherein tangentially applied forces produce a sliding or skewing type of deformation. Therefore, when shear is applied and distortion is allowed more accurate measurements are possible.
Analyses of soils and experiments in the simple shear devices showed that a region in the middle of the samples will be under uniform conditions of stresses and strains. However, instrumentation was not concentrated in this uniform region in routine simple shear devices. Rather the majority of simple shear results comes from measurements made over the plan area of the samples. These results, then, include apparatus effects of which the major one is stress concentrations. Thus, these results cannot be reliably used to describe the mechanical behavior of soils or other materials in simple shear.
Simple shear tests can be conducted on variations of 2 types of simple shear devices currently available; the Norwegian Geotechnical Institute (NGI) type (Bjerrum, L., and Landva, A., "Direct Simple Shear Tests on a Norwegian Quick Clay," Geotechnique, Vol. 16, No. 1, 1966, pp. 1-20 and Kjellman, W., "Testing the Shear Strength of Clays in Sweden," Geotechnique, Vol. 2, No. 3, pp. 225-232) and the Cambridge University (CU) type (Roscoe, K. H., "An Apparatus for the Application of Simple Shear to Soil Samples," Proc. 3rd ICSMFE, Vol. 1, 1953, pp. 186-191). Although both of these devices are unable to subject the sample to uniform states of stress and strain, there are important differences between them. One difference is the result of the geometry of the sample. Another difference is the measuring and determination of the stresses.
The NGI type accepts a cylindrical sample enclosed by a wire reinforced rubber membrane. The simplicity and ease with which field cores may be mounted in the NGI apparatus has made this model popular. In this device, however, only average shear and vertical stresses on the horizontal boundary can be determined. Some effort was directed to measuring the lateral stresses by using part of the wire reinforcement as a strain gage. However, it was found that the radial stresses were being measured. The radial stresses are neither equal to the lateral stress parallel to the direction of shearing nor the intermediate principal stress for simple shear stress state. Thus, it is not possible to use the measurements made in the NGI apparatus to compute the stress invariants which are necessary, at least, for the validation of mathematical models and for comparing the results of simple shear tests with other devices such as the triaxial device. The triaxial device is another common device for performing laboratory tests to study the stress-strain behavior of soils.
The CU type subjects a cuboidal sample to simple shear deformations imposed through the rotation of two hinged end flaps. The sample is surrounded by rigid boundaries and an array of load cells capable of measuring the normal and shear loads. From these measurements, the complete simple shear stress state of a mid-region of the sample can be computed. The CU apparatus has been used, largely in testing dry sands. The mounting and testing of clay samples in the device is very difficult. The area where the sample must be inserted must be dismantled before the sample is inserted and then reassembled. In addition, when a single series of clay tests was performed with a special version, the intermediate principal stress was not measured. The structure of the hinged end flaps also permits the seepage of water from the sample area leading to inaccurate measurements. Therefore, the routine testing of clays with this device is not feasible.
A common problem of many simple shear devices is the nonuniformity of stress/strain distribution within the soil specimen. A limiting factor in making measurements on soil samples has been the influence of the boundaries of the apparatuses, not found in the soil element in the field, on stress/strain distribution. This problem has been addressed in the past by the development of large-scale simple shear devices to overcome boundary influence. In addition, available devices could not identify and separately measure the lateral stresses .sigma..sub.x and .sigma..sub.z.
The present invention is suitable for routine testing of earthen materials including clays and sands, as well as powders. This device provides sufficient measurements from which the complete stress and strain states including the lateral stresses .sigma..sub.x and .sigma..sub.z can be determined. Although this device does not eliminate the nonuniformities, the instruments are located in a region of the sample where uniform conditions are likely to prevail so as to circumvent the non-uniformities which are known to be predominant at the ends of the samples.